In a conventional Joule effect glass heating furnace, a source of electrical energy is connected to a pair of powered electrodes immersed in a pool of molten glass. The molten glass acts as an electrical conductor, thereby permitting an electric current to flow therethrough between the powered electrodes. However, the molten glass also exhibits a certain amount of resistance to such current flow. As a result, the molten glass is heated as the current is passed therethrough. The total value of the electrical resistance experienced by the source of electrical energy is equal to the sum of the individual resistances of the molten glass and the two powered electrodes.
The resistance of the molten glass can be calculated as a function of the glass composition, the average temperature of the molten glass, and the length of the electrical path therethrough. For each of the powered electrodes, it is known that the electrical resistance thereof varies with its operating condition. For example, wear, erosion, internal cracks, and similar defects can change the effective surface area of a powered electrode and thereby alter the electrical resistance thereof. As the operating condition of a powered electrode changes, the flow of electrical current therethrough is also changed, assuming that the magnitude of the electrical energy supplied by the source remains constant. Thus, by measuring changes in the flow of electric current through the molten glass, information can be obtained regarding the operating condition of the pair of powered electrodes as they are used in the molten glass. Such information can alert an operator of the furnace of an impending failure of one of the electrodes, which can result in catastrophic damage to the furnace and injury to persons in the area.
Previous electrode resistance monitoring methods are known which sense electrical voltages and currents supplied to a powered electrode pair within a glass heating furnace. However, some of such known methods are subject to errors because of cross coupling with other electrode pairs in the furnace, which continue to be powered to heat the molten glass while measurements are being made. Additionally, such prior methods could not identify which of the two electrodes within a particular pair was defective. Other methods contemplate that the heating power be disconnected from the electrodes for a short period of time while measurement currents are passed between the electrodes. Unfortunately, disconnecting the heating power, even for a short time, may disrupt the flow of molten glass through the furnace, especially if the furnace is designed for relatively high flow rates.